1. Field of the Invention
The invention relates generally to optical sensing methods. More particularly, the invention relates to multi-photon optical sensing methods.
2. Description of the Related Art
The two-photon absorption (2PA) processes in semiconductor materials have been extensively studied both experimentally and theoretically, resulting in what are now well-established scaling rules that can accurately predict the degenerate two-photon absorption (D-2PA) of direct-gap semiconductor materials. These scaling rules show that D-2PA is inversely proportional to the cube of the energy gap, Eg. This means that the D-2PA coefficients in narrow-gap semiconductor materials are two to three orders of magnitude greater than the absorption in large gap semiconductor materials.
For example, ZnO (Eg=3.2 eV) has a D-2PA two-photon absorption coefficient, approximately 5 cm/GW at 532 nm, while for InSb (Eg=0.23 eV), approximately 2 cm/MW in the range 8 to 12 μm. 2PA coefficients in the cm/MW range may prove useful for practical applications, but these scaling rules imply that such large values are not accessible in the near infrared (NIR)/visible (VIS) range.
Since two-photon absorption processes within semiconductor materials are likely to continue to remain important within the context of several applications, desirable are additional semiconductor material based two-photon absorption processes that provide unique optical capabilities.